The present invention relates to a color cathode ray tube of the shadow mask type that has a magnetic shield for preventing for example color contamination due to deviation of the electron beam caused by an external magnetic field such as terrestrial magnetism.
In a color cathode ray tube with a built-in shadow mask, when the electron beam deviates under the influence of an external magnetic field, such as terrestrial magnetism, color contamination or other undesirable phenomenon occurs because undesired parts of a phosphor screen is made to emit light. To eliminate the effect of the external magnetic field, an internal magnetic shield is mounted along the inner wall of a funnel, adjacent the edges of a generally rectangular shadow mask.
FIG. 11 is a sectional view of such a color cathode ray tube in the prior art. In describing the color cathode ray tube, the term "Z-axis" is used to mean the tube axis which is perpendicular to the shadow mask and passing the center of the shadow mask, the term "X-axis" is used to mean the axis orthogonal to the Z-axis and parallel to the longer sides (horizontally extending sides, i.e., the top and the bottom sides) of the shadow mask, and the term "Y-axis" is used to mean the axis orthogonal to the Z-axis and the X-axis and parallel to the shorter sides (vertically extending sides , i.e., the right and the left sides) of the shadow mask. Moreover, the term "front" is used to mean the direction toward the face plate of the cathode ray tube, and the term "rear" is used to mean the direction toward the electron gun.
The cathode ray tube shown in FIG. 11 comprises a glass envelope 1 made up of a neck 1a, a funnel 1b and a face plate or front panel 1c. An electron gun 2 is disposed in the neck 1a, a phosphor screen 3 formed of phosphors emitting light of red, green and blue are provided on the inner surface of the front panel 1c in a mosaic fashion. A rectangular shadow mask 4 is disposed to face the phosphor screen 3, and is provided with apertures 6 for passage of electron beams 9, as shown in FIG. 12.
A rectangular frame 5 comprises a parallel part 5a and a normal part 5b. The parallel part 5a is flat, parallel with a skirt 1ca of the Front panel 1c and extends along the edges of the phosphor screen 3. The normal part 5b is flat, connected to the rear edge of the parallel part 5a (that side of the parallel part 5a which is facing the electron gun 2) and extends therefrom inward, normal to the axis, denoted by Z, of the tube 1.
The cross section of the rectangular frame 5 is therefore L-shaped.
The parallel part 5a has a width smaller than the dimension of the skirt 1ca in the direction of the tube axis Z so that it is accommodated in the skirt 1ca.
The edges of the shadow mask 4 are fixed, by means of welding or the like, to the sides 5a of the frame 5, so that the shadow mask 4 is reinforced.
Springs 7 have one end fixed to the parallel part 5a of the frame 5. The shadow mask 4, the frame 5 and the springs 7 together form a shadow mask structure 20.
The springs 7 have holes at their other ends engaged with pins (not shown) erected on the inner surface of the respective sides of the skirt 1ca, and the shadow mask 4 and the phosphor screen 3 face each other, being separated by a predetermined distance.
An inner magnetic shield 8 is formed from a thin sheet and formed to have a shape of a frustum of pyramid or prismold having side surfaces extending along the inner surface of the funnel 1b.
The periphery 8a of the front edge of the inner magnetic shield 8 are fixed, by means of welding or the like, to the rear side of the normal part 5b of the frame 5.
The electron beam 9 emitted from the electron gun 2 is scanned over the range indicated by chain lines 9a and 9b in FIG. 11, and the electron beam having passed the apertures 6 impinges on the phosphor screen 3 to cause selective light emission from the phosphors.
When the color cathode ray tube is in an environmental magnetic field, such as terrestrial magnetism, the path of the electron beam 9 is bent. Such influence of the environmental magnetic field is removed or suppressed by the inner magnetic shield 8. That is, in the space enclosed by the inner magnetic shield 8, the environmental magnetic field is reduced, so the bending of the path of the electron beam 9 is reduced, and the deviation in the position of impingement onto the phosphor screen 3 is reduced, and the color contamination is therefore reduced.
Observation of the magnetic shielding effect of the inner magnetic shield 8 reveals that when the color cathode ray tube is disposed to face the east or the west (disposed in the E direction or the W direction), the magnetic flux concentrates on the inner magnetic shield 8 and the frame 5, so the shield effect in the space within enclosed by these members is substantial.
When the color cathode ray tube is disposed to face the north or the south (disposed in the N direction or the S direction), the inner magnetic shield 8 is open in the direction of the phosphor screen 3, so the magnetic shield effect is less than when the cathode ray tube is in the E or W direction. Thus, the magnetic shielding effect is different between the E and W direction and N and S directions. In other words, the magnetic shielding effect has an anisotropy. It Is however desirable that the magnetic shielding effect is approximately equal in all directions. However, with the magnetic shield 8 in the shape of the frustum of pyramid, it is not possible to independently adjust the magnetic shield effect in the E and W directions, and the N and S directions, and the designing is made based on experience.
When the inner magnetic shield is not provided, the bending of the electron beam path due to the environmental magnetic field when the cathode ray tube is in the E or W direction is in the vertical direction. So, with a type of cathode ray tube having phosphor stripes elongated in vertical direction, the deviation of the electron beam impinging positions in the vertical direction do not cause serious color contamination. When however the magnetic shield 8 in the shape of a frustum of pyramid as shown in FIG. 11 is used (to improve the shield effect for the cathode ray tube in the N or S direction), if the cathode ray tube is in the E or W direction, the magnetic shield 8 not only gives the desired shielding effect, but also modifies the environmental magnetic field. The shield effect for the cathode ray tube in the E or W direction may therefore be inferior than if the magnetic shield 8 is not used at all. It often happens that the intended improvement in the magnetic shielding is not attained.
The terrestrial magnetism as an environmental magnetic field consists of a horizontal components and a vertical components, and the effect of vertical component is constant regardless of the direction in which the cathode ray tube is disposed. The vertical component is substantially constant throughout a substantial part of the world, so the effect of the vertical component can be easily eliminated by appropriate design. The effect of the horizontal component of the terrestrial magnetism is more difficult to overcome.
Since the prior art color cathode ray tube is configured as described above, the shielding effect by means of the magnetic shield is insufficient, and it is not possible to determine or alter the shielding effect independently for the E and W directions, and for the N and S directions. Moreover, the designing was made by experience, and was time-consuming.